It speaks powerfully about the changing relationship between two key allies in a Middle East where the strategic landscape has been transformed by America's invasion of Iraq.

The warnings from Turkey could not be clearer. If the full House of Representatives in Washington votes to back the labelling of the mass killing of Armenians as genocide, then serious consequences will follow.

The handling of the 1915 killings is still extremely sensitive

This could mean, for example, denying the US military the ability to ship crucial supplies through Turkish bases for operations in Iraq.

The fact that President George Bush publicly urged Congress not to proceed with the issue seems to have had little impact either at home or abroad. And that this would be a non-binding resolution, implying no practical shift in US policy, seems to make little difference to Turkish opinion either.

It is clear the Armenian massacres are a hugely sensitive issue for Turkey. Debate has raged on this issue, often prompting diplomatic strains. It has been a factor complicating ties, for example, between Ankara and Paris.

Complicated relationship

But the strains between the US and Turkey arise from the confluence of a number of factors.

Things are made worse by the fact that this row is unfolding in a very different context from that which characterised the generally stable relationship between Washington and Ankara during the Cold War years.

The initial crunch in US-Turkish ties came in the run-up to the US invasion of Iraq

Then, Turkey anchored the Atlantic Alliance's southern flank against attack from the Soviet Union. In many ways the relationship was simple.

Today, it is much more complex, not least because of the political transformation that has taken place inside Turkey. The country's secular-minded generals now play an important, but less central, role in day-to-day governance, and a moderate Islamist-rooted party has taken the democratic path to power.

The initial crunch in US-Turkish ties came in the run-up to the US invasion of Iraq, when the Turkish parliament refused to allow Turkish territory to be the staging post for the operation to topple Saddam Hussein's regime.

Since then both sides have tried to repair the damage, with the Americans, for example, applying huge diplomatic pressure to encourage some of its more reluctant allies to facilitate Turkey's membership of the European Union.

The removal of a strong Iraq from the Middle East's political chessboard has, though, greatly changed the regional dynamics. It has served to accentuate Turkish aspirations of becoming a key diplomatic player.

Feeling partially rebuffed by the Europeans, given the tortuous process of EU accession, Turkey is seeking new ties and new allies in the Middle East.

Its overtures have not been hampered by the fact that it still maintains reasonably close ties with Israel. Indeed that country has helped Turkey to take on something of a mediating role.

Kurdish factor

But the collapse of strong central authority in Iraq has also provided another looming problem with Washington.

The last thing that Turkey wants to see is an independent Kurdish state in northern Iraq which it believes would create wider instability. But it also wants something done about Kurdish guerrillas - PKK fighters - who continue to cross over the border to attack Turkish troops.

In the wake of recent incidents there are now growing fears of a Turkish military incursion into northern Iraq to neutralise Kurdish separatist guerrillas who have their camps there.

The pressures on the Turkish authorities to act are growing. The Turkish army has stepped up its bombardment of targets in northern Iraq.

Officials in Baghdad and Washington are alarmed. This is a new element in the Iraqi drama that the Americans want to avoid at all costs.